Current collector structure

ABSTRACT

A current collector structure that is used to distribute an electrical potential to an electrode layer of an electrochemical cell. The electrochemical cell has an electrolyte layer adjacent the electrode layer for transport of oxygen ions. The current collector structure has a porous, electrically conductive layer located on the electrode layer. Additionally, at least one elongated strip-like layer formed of an electrically conductive material is located on the porous, electrically conductive layer, opposite the electrode and oriented in a lengthwise direction of the electrode layer.

FIELD OF THE INVENTION

The present invention relates to a current collector structure to distribute an electrical potential to an electrode layer of an electrochemical cell. More particularly, the present invention relates to such a current collector structure in which a porous, electrically conductive layer is located on the electrode layer and one or more elongated strip-like layers formed of an electrically conductive material are located on the porous, electrically conductive layer.

BACKGROUND OF THE INVENTION

Electrochemical cells are used for separating oxygen from an oxygen containing feed. The purpose of the separation can be to concentrate oxygen or to remove oxygen from the feed for purification purposes.

The electrochemical cell has an electrolyte layer to conduct oxygen ions that is located between two electrode layers to apply an electrical potential across the electrolyte. The electrode layers are porous and can have sublayers while the electrolyte is an air-tight, dense layer. The resulting composite structure can be in the form of a tube in which the oxygen containing feed is fed to the inside of the tube and the separated oxygen is either collected on the outside of the tube and then dissipated. The reverse is possible and oxygen can be fed to the outside of the tube and the permeated oxygen collected on the inside of the tube. Other forms are possible, for example, flat plates and honeycomb-like structures.

The electrolyte layer is formed of an ionic conductor that is capable of conducting oxygen ions when subjected to an elevated temperature and an electrical potential applied to the electrode layers. Under such circumstances, the oxygen ions will ionize on one surface of the electrolyte layer and under the impetus of the electrical potential will be transported through the electrolyte layer to the opposite side where the oxygen ions will recombine into molecular oxygen. Typical materials that are used to form the electrolyte layer are yttria stabilized zirconia and gadolinium doped ceria. The electrical potential is applied to the electrolyte by way of cathode and anode electrodes. The oxygen ionizes at the cathode and the oxygen ions recombine at the anode. Typically, electrodes can be made of mixtures of the electrolyte material and a conductive metal, metal alloy or an electrically conductive perovskite.

In order to distribute current to the electrodes, current collectors are utilized in the form of layers on the electrodes opposite to the electrolyte. In some electrochemical cells, only the cathode electrode layer is provided with a current collector layer. In other electrochemical cells, both the cathode and the anode layers are provided with current collectors. However, it is important that the current collector layer also be porous to allow for oxygen transport through the current collector layer to the electrode layer for ionization purposes. The current collector layer is made as thin as possible in order to minimize the transport resistance. However, this increases the electrical resistance of the current collector layer.

U.S. Pat. No. 6,457,657 discloses a porous current collector in which a current collector is formed of a mixture of ceramic particles and metal conductive particles to improve the aging characteristics of the current collector. In this regard, where current collectors are made of metals and metal alloys, given the operational temperatures of electrochemical cells, the pores within such structures, over time, tend to close to decrease the oxygen transport through the current collector.

Other current collectors are not formed of layers. For example, in U.S. Patent Appln. Serial No. 2007/0003818 A1, a spring loaded wire current collector is utilized that contacts the electrode. In “High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications”, Elsevier, pp. 219-220 by Singhal et al. (2004) a wrapped wire current collector is disclosed. In “Cathode Current-Collectors for a Novel Tubular SOFC Design”, Journal of Power Sources 70, pp. 85-90 by Hatchwell et al. (1998) a silver strip and silver wire is utilized to distribute an electrical potential to an electrode.

As will be discussed, the present invention among other advantages provides a current collector structure that utilizes a porous, electrically conductive layer over the electrode layer so that an electrical potential can be distributed throughout the surface of the electrode layer. However, it overcomes the disadvantages of such a structure in which the resistance increases as such a current collector is attempted to be made sufficiently thin to minimize resistance to transport of molecular oxygen through such layer.

SUMMARY OF THE INVENTION

The present invention provides a current collector structure to distribute an electrical potential to an electrode layer of an electrochemical cell having an electrolyte layer adjacent the electrode layer for transport of oxygen ions. The current collector is provided with a porous, electrically conductive layer located on the electrode layer. At least one elongated strip-like layer formed of an electrically conductive material is situated on the porous, electrically conductive layer, opposite to the electrode and oriented in a lengthwise direction of the electrode layer.

The porous, electrically conductive layer can be formed of particles of a metal or metal alloy containing surface deposits of a metallic oxide. Preferably, the electrically conductive material and the metal or metal alloy is silver and the metallic oxide is zirconia. The silver within the at least one elongated strip-like layer can have a density of between about 50% and about 100% of the theoretical density of silver. In other words, the strip-like layer can be porous or dense without pores.

The elongated strip-like layer preferably has a thickness and a width that are each of between about 100 microns and about 1000 microns. Preferably, the porous, electrically conductive layer is between about 15 microns and about 25 microns thick.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:

FIG. 1 is a fragmentary, schematic sectional view of an electrochemical cell of tubular configuration having a current collector structure in accordance with the present invention;

FIG. 2 is a transverse cross-sectional view of FIG. 1; and

FIG. 3 is a fragmentary, electron micrograph of a current collector in accordance with the present invention applied to an electrode of an electrochemical cell.

DETAILED DESCRIPTION

With reference to FIGS. 1 and 2 an electrochemical cell 1 incorporating a current collector structure in accordance with the present invention is illustrated.

The electrochemical cell 1 has an electrolyte layer 10 that is formed of a ceramic that is an ionic conductor at high temperatures such as between about 400° C. and 1000° C. When the electrochemical cell 1 is subjected to a high temperature and an electrical potential is applied to opposite surfaces of the electrolyte layer 10 oxygen ions will be conducted through the electrolyte to produce an oxygen permeate. Thus, as would be well known in the art, the electrochemical cell would be located within an insulated enclosure in which the electrochemical cell 1 would be heated, fed with an oxygen containing stream 14 and from which an oxygen permeate stream 16 would be discharged.

In the illustrated embodiment, electrochemical cell 1 is of tubular form. As indicated above, other configurations are possible such as flat plates. An oxygen containing stream 14 is contacted with the exterior of electrochemical cell 14. Oxygen ions within the oxygen containing gas are conducted through electrolyte layer 10. The oxygen ions recombine in the interior of the tube to form an oxygen permeate stream designated by arrowhead 16. The electrical potential in the illustrated embodiment is applied to a cathode electrode layer 18 and an anode electrode layer 18 that are located on opposite sides of the electrolyte layer 10. The electrical potential 12 that is applied to the cathode electrode layer 18 and the anode electrode layer 18 serve as the driving force for oxygen ion transport.

The cathode electrode layer 18 and the anode electrode layer 18 can be formed of mixtures of metal or a metal alloy and the ceramic used in the electrolyte for thermal compatibility purposes. For example, the cathode electrode layer 18 and the anode electrode layer 20 could be formed of a mixture of yttria stabilized zirconia and silver particles for an electrolyte layer 10 formed of yttria stabilized zirconia. In case of an electrolyte layer 10 formed of gadolinium doped ceria, the cathode and anode electrodes 18 and 20 could be formed of a mixture that contains 65% by weight of lanthanum strontium iron cobalt oxide, remainder gadolinium doped ceria. In an illustrated example herein, electrolyte layer 10 is formed of scandia stabilized zirconia and the cathode and anode electrodes 18 and 20 are each formed of a mixture of about 60% by weight of lanthanum strontium manganate, remainder scandia stabilized zirconia. In order for the oxygen to reach the surface of the electrode 10, the cathode and anode electrode layers 18 and 20 are porous and can have a porosity of between about 25% and about 40% and a pore size of between about 0.1 and about 2.0 microns. The electrolyte layer 10 is a dense layer that has few pores and any pores that exist are not connected so that the electrolyte layer 10 is air tight. It is understood, however, that the present invention has application to any type of electrochemical cell of any architecture and formed from any suitable materials.

The current collector structure of the present invention has a porous, electrically conductive layer 22 located on the cathode electrode layer 18, and in the illustrated embodiment two strip-like layers 24 and 26, each located opposite to one another and opposite to the cathode electrode layer 18 and two strip-like layers 28 and 30 located on a porous, electrically conductive layer 32 located on the anode electrode layer 20. However, it is to be noted that there could be only one strip-like layer associated with each porous electrically conductive layer or there could be more than two of such strip-like layers depending on the size of the electrochemical cell 1.

As can be appreciated, the current collector structure as described above could be provided only on the cathode electrode layer 18 in certain applications known in the art. Additionally, the illustrated structure could be reversed with the cathode layer 20 situated at the inside of the tube and the anode layer 20 located on the exterior of the tube. In such case the oxygen containing gas 14 would be introduced into the tube.

The porous, electrically conductive layers 22 and 32 must be porous to allow oxygen to permeate through the cathode electrode layer 18 to the electrolyte layer 10. Although the porous, electrically conductive layer 22 or 32 could be formed of a metal or metal alloy or mixtures thereof or mixtures of a metal and a metallic oxide such as zirconia, preferred porous, electrically conductive layer 22 or 32 are formed from a powder containing a metal or metal alloy having surface deposits of a metallic oxide. Such a powder can be produced by methods well known in the art, for example by wash-coating or mechanical alloying. For exemplary purposes, a silver powder, designated as FERRO S11000-02 powder, was obtained from Ferro Corporation, Electronic Material Systems, 3900 South Clinton Avenue, South Plainfield, N.J. 07080 USA. The size of particles contained in such powder is between about 3 and about 10 microns in diameter and the particles have a low specific surface are of about 0.2 m²/gram. Zirconia surface deposits were formed on such powder such that the zirconia accounted for about 0.25% of the weight of the coated particle. As may be appreciated, other electrical conductive metals and metal alloys can be utilized, such as Au, Pd, Pt, Ni, Ru, Rh, Ir and alloys of two or more of such elements. Furthermore, the metallic oxide, in addition to zirconia, can be CeO₂, doped-ZrO₂ (e.g. yttria stabilized zirconia—YSZ), doped-CeO₂ (e.g. gadolinia doped ceria—CGO), Y₂O₃, Al₂O₃, Cr₂O₃, MoO₃, Nb₂O₅, TiO₂, Ta₂O₅, SnO₂, La_(0.6)Sr_(0.4)CO_(0.2)Fe_(0.8)O₃, La_(0.8)Sr_(0.2)MnO₃, La_(0.8)Sr_(0.2)FeO₃, La_(0.8)Sr_(0.2)CrO₃, or La_(0.8)Sr_(0.2)CoO₃.

While particle sizes for the metal or metal alloy can be greater than those noted above, preferably the particle sizes range from between about 0.1 microns and about 20 microns. Additionally, although 0.25% by weight is a particularly preferred content of the metallic oxide, greater amounts could be used provided that such amounts are not greater than the weight of metal or metal alloy used in forming the electrically conductive particles. In this regard, the metallic oxide content of the electrically conductive particles is preferably between about 0.05% and about 1.0% by weight. The aforesaid contents by weight will remain unchanged in the finished current collector layer in that after sintering the metal or metal alloy and the metal oxide utilized in forming such layer will be distributed through the layer.

Such powder can be applied to the cathode electrode layer 18 and the anode electrode layer 20 of a sintered form of a layered structure containing an electrolyte layer 10 and the cathode electrode layer 18 and the anode electrode layer 18 by way of slurry dipping application techniques. Other types of applications could be used such as aerosol applications, screen printing and tape casting. The slurry content is of course modified in a manner well known in the art to fit the particular type of application utilized. The sintered form can be produced by a variety of well known techniques such as extrusion, injection molding, isopressing and tape casting or a combination of such techniques. It is to be noted that it is possible that the layered structure be in an unsintered or green state. In such case, after application of the porous, electrically conductive layer 22 to the cathode electrode layer 18 and the porous, electrically conductive layer 32 to anode electrode layer 18, the coated structure would then be co-fired to sinter the composite structure.

In case of dip coating, a suitable slurry can be formed by known techniques such as by mixing the electrically conductive particles or powder with solvents, such as ethanol and toluene, a binder such as polyvinyl butyral and a plasticizer such as dibutyl phtalate. A dispersant, such as menhaden fish oil may optionally be mixed into the slurry. In case of the silver-coated particles obtained from Ferro Corporation as indicated above, the slurry can be made up in accordance with such manufacturer's recommendations, namely, mixing the conductive particles with FERRO B-73210 Tape Casting Binder System (available from Ferro Corporation set forth above), ethanol and toluene. The particles are between about 45% and about 75% by weight of the slurry. Additionally, the binder system is between about 20% and 50% by weight of the slurry, remainder equal parts of the ethanol and toluene. A preferred slurry is about 70% by weight of particles, 20% by weight of the binder system and remainder equal parts of the ethanol and toluene. Obviously, the lower the percentage of particles, the more times the form must be dipped to obtain a desired thickness. The layered structure can then be dipped into the slurry and then dried and heated to remove the solvent and burn out the organic component such as the binder and plasticizing agent. Further heating partially sinters the current collector layer and produces the necessary porous, electrically conductive layers 22 and 32. The porous, electrically conductive layer, either 22 or 32, formed in a manner outlined above, preferably is between about 15 microns and about 25 microns thick and has a porosity of between about 30% and about 50%. A pore size range from between about 1 micron and about 10 microns is particularly preferred. Pore size or more specifically, average pore diameter, is measured by known quantitative stereological line intersection analytical techniques. Although well known, a specific reference to such techniques and a description thereof can be found in Quantitative Stereology, by E. E. Underwood, Addison-Wesley Publishing Co., Reading Mass., (1970). It is to be noted that a content of electrically conductive particles of between about 45% by weight and about 75% by weight of the slurry is necessary to produce the aforesaid thickness range for the porous, electrically conductive layer 22.

As can be appreciated, the thickness, porosity and pore size can be varied by varying the make-up of the slurry, the number of dip coatings and the particle size. Furthermore, even in case the porous, electrically conductive layers 22 are 32 is formed of a metal such as silver or other metal or metal alloy or other suitable electrically conductive material mixtures, the resultant layer could have the thickness, porosity and pore size outlined above.

The elongated strip-like layers 24, 26, 28 and 30 can have a thickness and a width that are each of between about 50 microns and about 2000 microns and can be between about 50% and 100% of the theoretical density of silver. In case of 100% theoretical density, the elongated strip-like layers 14 and 26 are not porous. They can be formed from a slurry of FERRO SF4MA silver flake obtained from Ferro Corporation that is mixed with 21 gms of a binder obtainable from the Ferro Corporation as binder B73210. The mixture can be placed in a coated glass jar with 25 gms of zirconia milling media and rolled for about 8 hours. The elongated strip-like layers 24 and 26 can be applied along the length of the tube using a syringe. The elongated strip-like layers 28 and 30 can be formed by placing a tube formed of PTFE having a small lateral opening of about 1.3 mm within the composite structure that will form the electrochemical cell 1. As mixture of silver and binder can then be pumped into the composite structure and withdrawn. For example, a length of #16 PTFE tubing (Cole-Parmer) is inserted all the way into a single porous electrode and current collector coated electrolyte tube, with one end attached to a 30 ml syringe fitted with a 17 GA needle and filled with silver paste. As the electrolyte tube is pulled linearly at a rate of 18 seconds/foot the silver paste is dispensed in a thin silver stripe along the inside length of the tube, using a 40 psi extrusion pressure setting. A silver stripe is applied on the outside of the electrolyte tube coated with a porous electrode and silver current collector in the same way, using the PTFE tubing held through the needle hole of a clamped syringe.

After application, the resulting composite structure containing the resulting strip-like layers is then fired at about 850° C. to adhere the strip-like layers to the underlying electrically conductive porous layer 22 and the electrically conductive porous layer 32 to produce the strip-like layers 24, 26, 28 and 30.

With reference to FIG. 3, an electrochemical cell 1 of the type described above was formed in which each of the cathode electrode layer 18 and the anode layer 20 were formed from a composite of about 60% by weight lanthanum strontium manganate, remainder scandia stabilized zirconia and the electrolyte layer 10 was formed of scandia stabilized zirconia. In the illustration, the electrolyte layer 10 is about 500 microns thick and each of the cathode and anode electrode layers 18 and 20 are about 40 microns thick. Each of the electrically conductive porous layers 22 and 32 is formed from silver particles having surface deposits of zirconia of about 0.25% by weight, the particles having a size of about 7 microns. The electrically conductive porous layers 22 and 32 had a thickness of about 20 microns, a pore size of about 12 microns and a porosity of about 50 volume %. Each of the strip-like layers 24, 26, 28 and 30 were formed of silver in the manner outlined above and had a thickness and width of about 300 micron and about 800 microns, respectively.

While the present invention has been described with reference to a preferred embodiment, as can be appreciated by those skilled in the art, numerous additions, omissions and changes can be made without departing from the spirit and the scope of the present invention that is set forth in the appended claims. 

1. A current collector structure to distribute an electrical potential to an electrode layer of an electrochemical cell having an electrolyte layer adjacent the electrode layer for transport of oxygen ions, said current collector structure comprising: a porous, electrically conductive layer located on the electrode layer; and at least one elongated strip-like layer formed of electrically conductive material located on the porous, electrically conductive layer, opposite to the electrode and oriented in a lengthwise direction of the electrode layer.
 2. The current collector structure of claim 1, wherein the porous, electrically conductive layer is formed of particles of a metal or metal alloy containing surface deposits of a metallic oxide.
 3. The current collector structure of claim 2, wherein the electrically conductive material and the metal or metal alloy is silver and the metallic oxide is zirconia.
 4. The current collector structure of claim 3, wherein the silver within the at least one elongated strip-like layer is between about 50% and about 100% the theoretical density of the silver.
 5. The current collector structure of claim 4, wherein the at least one elongated strip-like layer has a thickness and a width that are each between about 50 microns and about 2000 microns.
 6. The current collector structure of claim 5, wherein the porous, electrically conductive layer is between about 15 microns and about 25 microns thick. 